The present invention relates to an image forming apparatus having a combination or superimposed copying capability and a two-side or duplex copying capability and, more particularly, to a copier operable in a superimposed copy mode and a duplex copy mode and free from malfunctions ascribable to the failure of a sensor which is responsive to a paper jam.
Many of modern copiers are selectively operable in a combination or superimposed copy mode for reproducing information printed on a plurality of documents on one side of a single paper sheet, and a or duplex mode for reproducing them on opposite sides of a single paper sheet. A copier of this type has an exclusive paper transport path for implementing the superimposed and/or duplex copy mode in addition to a paper transport path for an ordinary one-side mode. The exclusive path branches off the ordinary path at a position between a discharge roller and a fixing device. The exclusive path extends to a register roller so as to refeed a paper sheet carrying an image on one side thereof, or one-sided paper sheet, to a photoconductive drum.
Regarding a paper transport mechanism, a copier with such superimposed copying and duplex copying capabilities is far more complicated and, therefore, needs more sensors responsive to the position of a paper sheet than an ordinary copier having a one-side copying capability only. The intricacy of paper transport mechanism directly translates into an increase in the frequency of paper jam, while an increase in the number of sensors results in an increase in the probability of their failure and, therefore, in false paper detection. For example, despite that no jam has occurred on the paper transport path, i.e., no paper sheet is present in the paper transport route, a sensor may determine that a paper sheet is present and may thereby interrupt the operation of the copier while displaying a jam on the copier. Then, one has to search for a jamming paper sheet which does not exist by opening the door of the copier. This not only wastes time and labor but also makes one distrustful of the copier.